feat: add implement syntax skill

.github/skills/implement-syntax/SKILL.md

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+---
+name: implement-syntax
+description: Implement a syntax feature from a GitHub issue across the Tree-sitter grammar, generated parser artifacts, typed C# syntax tree, binder, and tests. Use for `/implement-syntax`, `$implement-syntax`, or any issue-driven addition or change to Expressif syntax.
+---
+
+# Implement Syntax
+
+Implement syntax features end to end while preserving the authored source in the concrete syntax tree (CST).
+
+## 1. Investigate
+
+1. Read the repository `AGENTS.md` and follow its issue, worktree, Git, and pull-request requirements.
+2. Resolve `<issue>` as an issue number or URL, then read the complete issue, including relevant comments and linked context.
+3. Inspect `grammar.js` around every affected construct.
+4. Trace the corresponding generated Tree-sitter nodes, typed C# syntax nodes, binder cases, and tests to learn the current conventions.
+5. Keep the change syntactic. Leave semantic or runtime validation downstream unless the issue explicitly requires it.
+
+## 2. Design the CST before editing
+
+Form explicit alternatives for the intended CST shape. For each viable choice, present:
+
+- Tree-sitter node name;
+- typed hierarchy, such as `ExpressionSyntax` or `ValueSyntax`;
+- named fields;
+- child nodes and their source order;
+- whether punctuation or other authored tokens appear in `Children`;
+- how `Text` and source ranges behave;
+- whether the source form remains distinct or is lowered into another construct.
+
+Recommend one choice and explain the material tradeoffs. Prefer a lossless CST: preserve shorthand and specialized authored syntax instead of prematurely lowering it to function calls or another semantic representation.
+
+Pause and ask the user to confirm the CST design. Do not modify code, generated artifacts, or tests until the user confirms a choice. If the user changes the design, revise the proposal and confirm it again.
+
+## 3. Implement the confirmed design
+
+After confirmation:
+
+1. Modify `grammar.js`.
+2. Regenerate Tree-sitter artifacts with the repository's generation command. Never hand-edit generated files.
+3. Update the typed C# model:
+   - add the `SyntaxKind` member;
+   - add or update the appropriate `*Syntax` node;
+   - use the confirmed inheritance;
+   - expose clear public semantic properties.
+4. Update the C# binder for every new Tree-sitter node. Do not leave raw new node kinds unbound.
+5. Treat the feature as incomplete until the typed C# syntax tree exposes it cleanly.
+
+## 4. Test the complete syntax surface
+
+Add focused tests for every applicable behavior:
+
+- standalone syntax;
+- composition as an argument, pipeline element, or compound construct;
+- exact node type and `SyntaxKind`;
+- public properties;
+- `Children`, including order;
+- `Text` and preservation of authored syntax;
+- source spans or ranges;
+- malformed or incomplete input and recovery behavior.
+
+Use the confirmed CST design as the test contract. Avoid asserting semantic or runtime rules unless the issue explicitly includes them.
+
+## 5. Validate and deliver
+
+1. Run Tree-sitter generation validation and the Tree-sitter test suite using the repository commands.
+2. Run the relevant .NET tests, expanding to the full .NET test suite when practical.
+3. Review generated diffs to verify they result only from regeneration.
+4. Confirm every new Tree-sitter node has a typed binder path and every public typed property is tested.
+5. Complete the repository's required commit, push, issue-label, pull-request, and clean-worktree workflow.
+6. Report the CST decision, changed layers, validation commands and results, and pull request.

.github/skills/implement-syntax/agents/openai.yaml

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+interface:
+  display_name: "Implement Syntax"
+  short_description: "Implement grammar and typed CST features"
+  default_prompt: "Use $implement-syntax to implement syntax from the specified issue."